1. Field of the invention
The present invention relates to a superconducting material and a method for preparing the same. More particularly, it relates to a novel superconducting material composed of compound oxide having a higher critical temperature and a method for preparing the same.
2. Description of the related art
Under the phenomenon of superconductivity, the perfect diamagnetism is observed and no difference in potential is observed for all that an electric current of a constant finite value is observed internally.
The superconductivity can be utilized in the field of power electric applications such as MHD power generation, fusion power generation, power transmission, electric power reservation or the like; in the field of transportation for example magnetic levitation trains, magnetically propelling ships or the like; in the medical field such as high-energy beam radiation unit; in the field of science such as NMR or high-energy physics; a high sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like, or in the field of fusion power generation. In addition to the abovementioned power electric applications, the superconducting materials can be used in the field of electronics, for example, as a device using the Josephson device which is expected to be a high-speed and low-power consuming switching device.
The phenomenon of superconductivity, however, is observed only at very low cryogenic temperatures. In fact, a relatively lower temperature of 23.2 K which was the critical temperature (Tc) of a superconductor composed of Nb.sub.3 Ge have been the top record of the critical temperature among known superconducting materials.
This means that liquidized helium (boiling point of 4.2 K) is only one cryogen which can realize such very low temperature of Tc. However, helium is not only a limited costly resource but also require a large-scaled system for liquefaction. Therefore, there had been a strong demand for another superconducting materials having higher Tc. But no material which exceeded the abovementioned Tc had been found for all studies for the past ten years.
Possibility of existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986)189]. It was also reported in the news paper that C. W. Chu et al. discovered in the United States of America another superconducting material so called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature of about 90 K in February 1987. And hence, possibility of existence of high-temperature superconductors have burst on the scene.
It had been known that certain ceramics material of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba-Pb-Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba-Bi type compound oxides also show superconductivity. These superconductors, however, possess rather lower transition temperatures of about 10 K and hence usage of liquidized helium (boiling point of 4.2 K) as cryogen had been indispensable to realize superconductivity. Therefore, the abovementioned new type compound oxides in which superconductivity is realized in liquid nitrogen which is a relatively cheap cryogen will accelerate actual usage of superconductors.
An object of the present invention is to provide a new system of compound oxide which possess a higher critical temperature and a method for preparing the same.